Anal. Chem. 1994,66, 399-405
Comparison of Hydrodynamic Voltammetry Implemented by Sonication to a Rotating Disk Electrode Carolynne R. S. Hagan and Louis A. Coury, Jr.' Department of Chemistry, Duke University, Durham, North Carolina 27708-0346
Voltammetric and amperometric experimentsconducted during continuous sonication with a Ti immersion macrotip (1 cm2 area) positioned parallel to Pt working electrode surfaces are described. The effects of concentration, electrode area, temperature, kinematic viscosity, amplitude of vibration of the Ti sonicator tip, cell geometry and voltage scan rate on limiting currents are considered and compared to analogous measurements made at a rotating disk electrode. It is found that steadystate voltammograms may be obtained during sonication at scan rates up to 25 V/s. Hydrodynamic conditions are described in terms of the effective rotation rate which would be needed in rotating disk voltammetry to achieve similar transport rates. A procedure for performing sonovoltammetry in a cell geometry-independentmanner to probe the chemical effects of acoustic cavitation is discussed. A comprehensive investigation of the effects of ultrasound on electrochemical processes has been initiated in our laboratories with initial emphasis on mass transport behavior. During ultrasonic irradiation of solutions, rarefaction halfcycles associated with sound waves cause the formation of stabilized gas pockets at nucleation sites (e.g., liquid/solid interfaces, dust particles) through the process known as acoustic cavitation.' These bubbles increase in size, oscillate, and finally violently implode as a result of high external pressures associated with compression half-cycles. Luminescence experiments2 have shown that temperatures as high as 5000 K (for comparative purposes, roughly equal to the surface temperature of the sun3 ) and pressures of hundreds of atmospheres (pressures found in the deepest oceanic trenches3)are generated during cavitational collapse: a process which can also generate fluid microjets exhibiting velocities of over 100 m/s at extended surface^.^ However, since the duration of these extreme conditions is less than 1 P S , bulk ~ solution conditions do not change significantly on the time scale of many experimental techniques. Because electrode surfaces serve as efficient nucleation sites for cavitation, electrochemistry is uniquely suited to study the unusual physical and chemical phenomena occurring during sonication. Previous published reports in this area include studies of high-speed coulometry,6 hydrodynamic modulation voltammetry during pulsed operation of a microtip,' continuous sonication in a cleaning bath,a and effects (1) Atchley, A. A.; Crum, L. A. In Ultrasound: Its Chemical, Physical, and BiologicalEffecrs;Suslick, K. S., Ed.;VCH, Inc.: New York, 1988; pp 1-64. (2) Flint, E. B.; Suslick, K. S. Science 1991, 253, 1397-1399. (3) Suslick, K. S. Science 1990, 247, 1439-1445. (4) Suslick, K. S.; Doktycz, S. J.; Flint, E. E. Ultrasonics 1990, 28, 280-290. (5) Suslick, K. S.;Doktycz. S. J. Adu. Sonochem. 1990, I , 197-230, and references therein. (6) Bard, A. J. Anal. Chem. 1963, 35, 1125-1128.
0003-2700/94/0366-0399$04.50/0 0 1994 American Chemlcai Society
of cavitation on transport during electrode depassi~ation.~ The present work differs from all previous studies in that the electrode is positioned in the center of the cavitational plume generated during continuous irradiation of a small volume of solution (1 50 mL) with high-intensity ultrasound generated by a parallel titanium macro-tip (area 1 cm2). Theseconditions were selected to be analogous to those usually chosen for (nonelectrochemical) sonochemistry studies.2-5 Furthermore, the sonochemicalequipment used here is of commercial design and also of the type most widely employed in the sonochemical ~ o m r n u n i t y , making ~ - ~ direct comparisons to work from other laboratories feasible. One of the best-developed hydrodynamic electroanalytical techniques is rotating disk electrode (rde) voltammetry, in terms of both theory and applications. In order to provide a frame of reference for the results of sonovoltammetric experiments reported here, comparisons will be made to analogousmeasurements at a rde.8 In particular, sonochemical currents will be expressed in terms of the rotation rate that would theoretically be needed in a laminar flow rde experiment to yield the same response. It is of general importance to fully understand the effects of ultrasound on interfacial mass transport before other electrochemical phenomena of interest such as altered rates of electron transfer during sonication may be addressed.
EXPERI MENTAL SECTION Electrochemical experiments were performed using a BASlOOB electrochemical analyzer (Bioanalytical Systems, Inc.; BAS) linked to a 386 personal computer. Anti-aliasing studies employed the built-in third-order low-pass filter of this instrument. Areas of microelectrodes were determined by linear sweepvoltammetry using a BAS PA- 1/C-2 low-current amplifier. A Heat Systems ultrasonic processor (Model XL2010, 475 W, tapped titanium horn, tip area 1 cm2) operating at 20 kHz was employed in sonoelectrochemical experiments. The amplitude of longitudinal vibration of the titanium amplifying horn was controlled by the generator output setting and varied from 0 to 120 pm peak to peak. Temperature control was maintained by a Brinkmann Lauda RM-6 refrigerated recirculating bath filled with ethylene glycol coolant. Temperature measurements were made with a Fluke 5 1 digital thermometer and K-type thermocouple, the tip of which was coated with silicone rubber (Permatex Color Guard) to prevent corrosion in the aqueous electrolyte solutions. Scanning electron micrographs (SEM) were acquired with a Philips 501 electron microscope. (7) Dewald, H. D.; Peterson, E. A. Anal. Chem. 1990,62, 779-782. (8) Huck, H. Ber. Bunsenges. Phys. Chem. 1987, 91,648-654. (9) Perusich, S. A.; Alkire, R. C. J . Electrochem. SOC.1991, 138,
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Analyncel Chemistry, Vol. 66,No. 3,February 1, 1994 599
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0.09 0.06 0.02 -0.02
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E/mV Figure 1. Sonoelectrochemical cell: (a) titanium amplifying horn, (b) cavttational plume, (c) Pt disk working electrode, (d) Ag/AgCI reference electrode, (e)W auxiliaryelectrode, (f) fine porosity glass frlt, (9) coolant inlet, and (h) coolant outlet.
Flgure 2. Vottammetry in the presence of (upper curve) and absence of (lower) sonication: [Fe(CN)e3-’C ] = 9.78 mM in 3 M KCI; E vs Ag/AgCI; vlbrational amplitude 120 pm; u = 25 mV/s; T = 23 i 2 O C ; horn tip/electrode separation distance 1.5 mm; horn tip/ceil floor separation distance 16 mm.
currents monitored. Examination of such current differences corrects for changes in the ratio of bulk concentrations of the oxidized-to-reduced form of the redox couple resulting from successive experiments with the same solution at the high mass transport rates produced. In other experiments linear sweep voltammetry (LSV) was used, and limiting current values were obtained as the difference between plateau and base currents. Kinematic viscosities of sucrose solutions were determined from a calibration curve ( R = 0.991) of efflux time vs kinematic viscosity generated from data for a variety of solvents at 22 oc using a Canon viscometer.
All potassium ferricyanide (Fisher) and potassium ferrocyanide (Aldrich) solutions were prepared in 3 M KCl (Mallinckrodt) and stored in the dark. Solutions in the concentration studies were prepared with deionized water obtained from a low-pressure reverse osmosis system (Barnstead ROpure LP) in conjunction with a Barnstead NANOpure ultrapure water system which yielded a product with a minimum resistivity of 18.1 MO cm. Departmentally deionized water was used in the other experiments. The areas of the platinum macroelectrodes (BAS) used for the majority of these studies were determined by chronocoulometry to be 0.0250 and 0.0255 cm2. Microelectrodes were constructed by heat-sealing Pt microwire of various diameters inside glass tubing. Reference electrodes were made by depositing a silver chloride coating onto silver wire (Aldrich). Palladium wire (Aldrich) served as the auxiliary electrode. The 150-mLjacketed electrochemical cell was constructed such that the surface of the working electrode was held securely through a threaded bushing directly below and parallel to the titanium amplifying horn and was, therefore, positioned into the most intense region of the cavitational plume (Figure 1). Uncertainties associated with separation distances specified are estimated to be
